Sounds like the PSU is at fault, or atleast a likely suspect.
Try removing the RAM and attempting to start it again, the mobo should beep at you saying there's no RAM. If it doesnt, then either the Mobo or PSU is at fault.
To test the PSU, easiest thing to do is get another one thats known to work and replace it. If you get the same result, its not the PSU's fault.
If the motherboard is at fault, there isn't much you can do except to just RMA it and get another.